This invention relates to variable magnification optical systems in which a front lens component having negative focal length is movable, for focusing, relative to a rear lens component having positive focal length, and an axial air separation between the front and rear components is variable to effect variation of the focal length of the overall optical system, and more particularly to an improvement of such an optical system which provides for correction of aberrations throughout the zooming and focusing ranges.
FIG. 1 schematically shows a mechanical-compensated variable magnification objective optical system to which the present invention is applicable. From front to rear in the direction in which light enters the optical system from the object end, the system is essentially composed of a first lens component 1 of negative refractivity and a second lens component 2 of positive refractivity. Both components are axially movable in differential relation to each other and to a focal plane 3 at which an image of continuously variable size of an object located at a fixed distance from the system is formed. This variable focus objective lens is, because of its being of inverted telephoto type, advantageous for increasing the back focal length and the image angle and therefore suited for use in a still camera and also as a super wide angle zoom lens for a cinematographic camera and a television camera.
To focus the aforesaid type variable focus objective lens from an infinitely distant object to a close object, an independent forward axial movement is imparted to the first lens component 1 alone. With the varifocal objective lens having this focusing provision set for an object at a finite distance, however, it is proven that the zooming movement of the first and second components results in some image shift due to the introduction of the focusing movement of the first component to the zooming movement. Although the amount of image shift is usually so small as to fall within the depth of field, this conventional focusing method gives rise to an aberrational problem which becomes serious when the lens system is focused down to an extremely short object distance. This aberrational problem is generally encountered in a lens system consisting of a divergent front and a convergent rear lens component to increase the image angle thereof, as the astigmatism is increased with increase in the axial air separation between the front and rear components to result in a large degree of over-correction of field curvature. In varifocal objective lenses of the type described, therefore, the forward movement of the first lens component 1 for focusing to shorter object distances causes an increase in the axial air separation between the first and second components 1 and 2, resulting in an increased variation of astigmatism during zooming. This is true, though to lesser extent, in an alternative focusing method in which all the zoom control components namely, components 1 and 2 are axially moved in unison relative to the focal plane 3 to effect focusing. Even in this latter method, however, the astigmatism tends to vary to a considerable extent during zooming with the resulting field curvature being over-corrected.
It is known that in order to achieve good stabilization of astigmatism and field curvature throughout the focusing range, it is required to fulfill conditions which will be explained in connection with the following formulae for an object at a finite distance expressed in terms of third-order aberration coefficient. EQU III' = III - Q(V + II.sup.s) + Q.sup.2 I.sup.2 EQU iv' = iv - q(v + ii.sup.s) + Q.sup.2 I.sup.s
wherein
I: the spherical aberration coefficient PA1 II: the coma coefficient PA1 III: the astigmatism PA1 IV: the sagittal field curvature coefficient PA1 V: the distortion coefficient PA1 I.sup.s, II.sup.2 : the pupil aberration coefficients PA1 Q: the quantity dependent upon the object distance from the lens system
When these formulae are applied to the inverted telephoto type lens system, the final term Q.sup.2 I.sup.s may be treated as negligible because I.sup.s is almost zero, while the second term Q(V + II.sup.s) will have a finite value which is rendered more important when the object distance is decreased, because, although V and II.sup.s are usually positive and negative respectively, the absolute value of V is smaller than that of II.sup.s. Q is negative so that III' &lt; III; and IV' &lt; IV. As a result, the field curvature is over-corrected for an object at a finite distance. A requirement that variation of astigmatism and field curvature be reduced to zero during focusing can be fulfilled when the factor (V + II.sup.s) is zero. In the case of the inverted telephoto type lens, however, it is impossible to realize II.sup.s = 0, because the principal plane is situated ahead from the pupil plane with the result that the angle of incidence of a pupil-paraxial ray is larger than the angle of emergence thereof.
According to the prior art, therefore, ordinary variable magnification objective lenses of the inverted telephoto type have been designed unavoidably to provide astigmatism and field curvature which are very perceptible when focused down to short object distances. Moreover, this drawback is intensified by increasing the fack focal length. With a conventional variable magnification objective lens having focusing provision at the front lens component thereof, it is difficult to assure a completely uniform high quality imaging capability, particularly when the provision for a field of wide angle is made, due to the fact that an increasingly large part of an extra-axial pencil of rays is blocked by the peripheral edge of the first lens component as the focusing front component is moved forwards. For this reason, it has heretofore been difficult to decrease the object distance to which focusing is effected to a minimum.